In many wireless antenna applications it is desirable to receive or transmit signals from a wide variety of possible angles. However, the radiation pattern of an antenna element is never completely omni-directional, as there is always a direction from which an antenna receives less power than its optimal direction. For straight wire antennas, such as monopole or dipole antennas, the radiation pattern is zero in the direction of the wire.
Various attempts have been made to combine monopole and dipole antennas so as to create composite antennas that can transmit or receive from more directions with a more even power distribution. The ideal is generally to create a hemispherical radiation pattern for an antenna over a ground plane. However, the combination of a single monopole and dipole do not produce a radiation pattern that is very hemispherical as there are multiple local minima. In addition, collocation of the monopole and dipole is generally a problem and many previous attempts to combine monopoles and dipoles are sub-optimal because they are not accurately collocated.
WO2013109173A1 discusses a combined monopole and dipole antenna. A dipole antenna has a common-mode rejection filter positioned along a non-shielded portion of the dipole transmission line so as to create an orthogonal monopole element from the non-shielded transmission line. Although this disclosure addresses the collocation problem, the described antenna requires a common-mode rejection filter and the resultant complexities which that entails. The non-shielded transmission line may also result in spurious interference even when driven by differential mode excitation.
The invention aims to address these and other shortcomings, at least to some extent.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.